The long term goal of this project is to understand how the antigen processing mechanism generates antigenic peptide/MHC class I complexes by unconventional (cryptic) translation. Recent studies, from several different laboratories have demonstrated that naturally processed peptides presented by MHC class I molecules on the surface of tumors, virally infected cells, and transfected cells can arise from regions of messenger RNA that were not expected to be translated. The mechanism by which cells sample this cryptic source of antigenic peptides versus the conventional source representing by-products of protein turn-over is not known. In addition, to what extent these cryptic peptides contribute to the diverse pool of peptides presented by MHC molecules and their immunological role remains to be established. In this proposal we will test the hypotheses that these unconventional, cryptic peptides could be products of a novel translational mechanism and could play a significant immunological role in normal animals. Using unique reagents and methods established in this laboratory these hypotheses will be tested by, (1) defining the context rules which determine how cryptic translation of the mRNA occurs and results in expression of peptide/MHC class I complexes, (2) determining the immunological role of cryptic peptide/MHC class I complexes in normal transgenic mice, and, (3) analyzing the different steps in the pathway for generation of cryptic peptide/MHC class I complexes. It is expected that improved understanding of the mechanism for generation of cryptic peptide/MHC complexes will not only provide novel sources of antigenic peptides and approaches to vaccine design but will also yield new insights into protein translation and immunesurveillance mechanisms.